This invention relates to a method for preparing an alloy for bonded Sm2Co17 base magnets and a bonded Sm2Co17 base magnet composition comprising the alloy.
Bonded Sm2Co17 base magnet powder is traditionally prepared by milling an alloy ingot having a regulated composition to a particle size of about 1 to 10 microns, pressing and shaping the resulting powder in a magnetic field to form a powder compact, sintering the powder compact in an argon atmosphere at 1100 to 1300xc2x0 C., and typically about 1200xc2x0 C., for a time of 1 to 5 hours, followed by solution treatment. Next, the solution-treated compact is subjected to aging treatment in which it is held at a temperature of 700 to 900xc2x0 C., and typically about 800xc2x0 C., for about 10 hours, then gradually cooled to 400xc2x0 C. or lower at a descending rate of xe2x88x921.0xc2x0 C./min. The sintered magnet is finally ground to a predetermined particle size. This powder metallurgy process, however, requires a more number of steps and a longer time than the sintered magnet producing process, and has the drawbacks of increased cost and low production efficiency.
In another traditional process, bonded Sm2Co17 base magnet powder is prepared by subjecting an alloy ingot having a regulated composition to solution treatment in an argon atmosphere at 1100 to 1300xc2x0 C., and typically about 1200xc2x0 C., followed by aging treatment in which it is held at a temperature of 700 to 900xc2x0 C., and typically about 800xc2x0 C., for about 10 hours, then gradually cooled to 400xc2x0 C. or lower at a rate of xe2x88x921.0xc2x0 C./min. The treated ingot is finally ground to a predetermined particle size. The bonded rare earth magnet-forming alloy powder obtained by this process has the advantage of low cost, as compared with the bonded rare earth magnet-forming alloy powder obtained by the powder metallurgy process (involving grinding the once sintered rare earth magnet). However, the bonded Sm2Co17 base magnet powder obtained by this process is known to have magnetic properties which are affected by the crystalline state of the ingot following melting. Specifically, the bonded Sm2Co17 base magnet powder obtained from an ingot whose crystalline state is predominantly composed of chill crystals and equiaxed crystals has poor magnetic properties, especially low coercivity, whereas the bonded Sm2Co17 base magnet powder obtained from an ingot whose crystalline state is predominantly composed of columnar crystals has good magnetic properties, especially high coercivity (see JP-A 56-102533 and JP-A 7-57909).
It then becomes a common practice to cast a molten alloy into a mold such as a box-shaped mold so that the macroscopic structure is composed of columnar crystals. Although the cooling rate of molten alloy must be increased to a certain level in order to obtain columnar crystals, the casting process using a box-shaped mold has the tendency that the cooling rate in a central portion of the ingot is lower than the cooling rate above which columnar crystals form, resulting in a coarse-grained structure and generation of equiaxed crystals. This problem can be overcome by such means as reducing the thickness of an ingot or increasing the surface area of the mold in contact with the molten alloy (see JP-A 4-146604 and JP-A 4-152603). Since these means sacrifice production efficiency, an ingot of a certain thickness must be furnished, which often results in a coarse-grained structure and generation of equiaxed crystals. It is then difficult to obtain at the end of casting an ingot which is predominantly composed of columnar crystals. The coarse-grained structure and the generation of equiaxed crystals are the main reason why satisfactory magnetic properties are not available in the bonded Sm2Co17 base magnet powder.
One solution to the above problem is a casting technique using a single roll, known as strip casting technique, which results in more than 90% by volume of columnar crystals (see JP-A 8-260083). The ingot produced by this casting technique has a microcrystalline structure and a uniform alloy structure free of segregation. In the case of anisotropic bonded rare earth magnets, however, anisotropic bonded rare earth magnets having satisfactory magnetic properties cannot be manufactured unless all bonded rare earth magnet-forming powder particles are unidirectionally oriented. Since the ingot obtained by the strip casting technique has a microcrystalline structure, the bonded rare earth magnet-forming alloy must be ground into a fine powder or the ingot must be subjected to heat treatment and solution treatment to induce grain growth. However, in the former case wherein the bonded rare earth magnet-forming alloy is ground into a fine powder, a compositional shift readily occurs because of the susceptibility of fine particles to oxidation. There can be even the danger of ignition by instantaneous oxidation. Additionally, bonded magnets produced from the powder fail to have a sufficient packing density and satisfactory magnetic properties. In the latter case wherein the ingot obtained by the strip casting technique is subjected to heat treatment and solution treatment, since the ingot is in the form of flakes and thus has a large surface area, an extended period of solution treatment can cause the ingot to be degraded by leakage in the heat treating furnace, and Sm in the ingot to evaporate off. Then satisfactory magnetic properties are not obtained as well.
An object of the invention is to provide a method for preparing a bonded Sm2Co17 base magnet-forming alloy having improved magnetic properties; and a bonded magnet composition comprising the bonded Sm2Co17 base magnet-forming alloy and having improved magnetic properties.
The inventor studied the relationship of the structure of a Sm2Co17 base alloy to a structural change by heat treatment. It has been discovered that when a Sm2Co17 base alloy containing at least 20% by volume of equiaxed crystals with a grain size of 1 to 200 xcexcm and having a strip gage of 0.05 to 3 mm or a Sm2Co17 base alloy obtained by quenching a corresponding alloy melt from a melt temperature of 1250 to 1600xc2x0 C. by a strip casting technique is used, a homogeneous structure can be accomplished by a brief duration of heat treatment. By heat treating the alloy in a non-oxidizing atmosphere under the conditions specified below for allowing the average grain size to grow up, there are achieved magnetic properties which are improved over the manufacture of a bonded Sm2Co17 base magnet-forming alloy from a prior art cast ingot.
In one aspect, the invention provides a method for preparing an alloy for bonded rare earth magnets, comprising the steps of melting an alloy consisting essentially of 20 to 30% by weight of R which is samarium or a mixture of at least two rare earth elements (inclusive of Y) containing at least 50% by weight of samarium, 10 to 45% by weight of iron, 1 to 10% by weight of copper, 0.5 to 5% by weight of zirconium, and the balance of cobalt; quenching the melt by a strip casting technique, to form a rare earth alloy strip containing at least 20% by volume of equiaxed crystals with a grain size of 1 to 200 xcexcm and having a gage of 0.05 to 3 mm; heat treating the strip in a non-oxidizing atmosphere at 1000 to 1300xc2x0 C. for 0.5 to 20 hours; followed by aging treatment and grinding.
In another aspect, the invention provides a method for preparing an alloy for bonded rare earth magnets, comprising the steps of melting an alloy of the same composition as above; quenching the melt from a melt temperature of 1250 to 1600xc2x0 C. by a strip casting technique; heat treating the resulting rare earth alloy in a non-oxidizing atmosphere at 1000 to 1300xc2x0 C. for 0.5 to 20 hours; followed by aging treatment and grinding.
A further embodiment of the invention is a bonded rare earth magnet composition comprising the bonded rare earth magnet-forming alloy obtained by either of the above methods and 1 to 10% by weight of a resin.
In the manufacture of a bonded Sm2Co17 base magnet-forming alloy, when a Sm2Co17 base alloy is subjected to solution treatment at high temperature and for a long time, samarium evaporates off due to its very high vapor pressure, eventually inviting a compositional shift. As a result, the bonded rare earth magnet obtained therefrom suffers degradation of magnetic properties, and typically substantial variation of coercivity. On the other hand, if the temperature or time of solution treatment is reduced in order to avoid the evaporation of Sm, the heat treatment becomes less effective, resulting in declines of remanence and maximum energy product. In contrast, using a rare earth alloy containing at least 20% by volume of equiaxed crystals with a grain size of 1 to 200 xcexcm and having a strip gage of 0.05 to 3 mm, which has been quenched by the strip casting technique, the present invention permits optimum solution treatment to be accomplished within a brief time. The use of the specific alloy allows the crystal grain size to grow up without entailing a compositional shift. Then the sequence of solution treatment, aging treatment and grinding to an optimum particle size yields a bonded Sm2Co17 base magnet-forming powder having improved magnetic properties. Using this bonded rare earth magnet-forming alloy and a resin as the raw material, a bonded rare earth magnet having improved magnetic properties can be produced.